This invention relates generally to the field of cryogenic vacuum pumps, commonly referred to as "cryopumps." Cryopumps utilize pumping surfaces cooled to cryogenic temperatures by a cryogenic refrigerator to condense and absorb gases.
A known vertical-mount cryopump design is illustrated in FIG. 1. The cryopump 10 is joined at a flange 14 to a process chamber, such as a process chamber of a cluster tool for semiconductor wafer fabrication. The cryopump 10 is then used to remove gases from the process chamber. As is typical, the illustrated cryopump 10 features a pair of pumping surfaces (use of the singular term, "pumping surfaces," hereafter, is to be understood to include both a single surface and any number of additional surfaces). In the illustrated embodiment, the primary, lower-temperature pumping surface is in the form of an array of baffles 34. The array of baffles 34 is cooled to a temperature of about 4K to about 25K by the second stage 32 of a two-stage cryogenic refrigerator. The higher-temperature pumping surface includes a radiation shield 36 and a frontal array 38 in thermal contact with the radiation shield 36. The radiation shield 36 and frontal array 38 surround the lower-temperature array of baffles 34 and are cooled by the first stage 29 of the cryogenic refrigerator. Both pumping surfaces are contained within a vacuum vessel 12.
When the cryopump 10 is operating, gases with higher boiling points (e.g., water vapor) are condensed on the higher-temperature pumping surface. Gases with lower boiling points (e.g., nitrogen) pass through the frontal array 38 of the higher-temperature pumping surface to the lower-temperature pumping surface where they are condensed. Further, an adsorbent, such as charcoal or a molecular sieve, is typically attached to the lower-temperature surface (e.g., to the underside of the baffles 34) to remove gases with very low boiling points, such as hydrogen, helium and neon. The above-described condensation and adsorption produce a high vacuum in the vacuum vellel 11 and in the process chamber to which the cryopump 10 is mounted.
Once a high vacuum is established, a workpiece (e.g., a semiconductor wafer) can be moved into and out of the process chamber through partially-evacuated load locks. Each time the process chamber is opened, additional gases enter there through. These gases are then condensed onto the pumping surfaces, thereby maintaining the low-pressure conditions needed for processing the workpiece. In addition, processing gases that are introduced in the process chamber are also condensed onto the pumping surfaces.
After several days or weeks of continued processing, the gases that condense and absorb on the pumping surfaces begin to saturate the cryopump 10. The trapped gases are then released from the pumping surfaces via a regeneration procedure, whereby the cryopump 10 is temporarily shut down to allow the pumping surfaces to warm. As the surfaces warm, so do the gases condensed thereon, thereby facilitating the release of these gases. The released gases are then purged from the vacuum vessel, and cooldown of the cryopump 10 is repeated.